1. Field of the Invention
The present invention relates to a device for controlling a stability of a behavior of a vehicle such as an automobile, and more specifically, to such a device that stabilizes the running behavior of a vehicle by controlling a steering angle of wheels.
2. Description of Prior Art
When a vehicle is running on a road surface having different frictional coefficients, e.g. dried in one side and wet or frozen in the other side, an imbalance in braking or traction forces exerted on tires (road reaction forces) would arise between the left and right wheels of the vehicle, generating a yaw moment turning the vehicle head undesirably. Such an imbalance in braking or traction forces, deteriorating its running behavior (e.g. straight line stability), could be large especially during the execution of Traction control (TRC) and/or Anti-skid (ABS) control, because the forces on wheels are individually controlled to suppress excessive increase in slippage of the respective tires, depending upon different road conditions. Then, so far, there have been developed control devices for stabilizing a vehicle running behavior by reducing and/or canceling the effect of a yaw moment owing to a braking or traction force imbalance, especially under TRC and/or Anti-skid control.
One of examples of such vehicle stability control devices for a vehicle running behavior is disclosed in Japanese Patent No. 2540742, in which, during Anti-skid control, wheels are steered independently of a driver's steering (handling) action so as to generate a counter yaw moment opposite to, and canceling the yaw moment owing to a braking force imbalance between the left and right wheels on a vehicle. The counter yaw moment, determined based upon braking forces or pressures actually applied for the respective wheels, is generated through the automatic steering of the wheels in the direction of turning the vehicle head from the higher frictional side (the side on which the higher braking (backward) force is exerted) to the lower frictional side (the side on which the lower braking force is exerted). In order to accomplish this stability control, a vehicle is equipped with a steering apparatus or mechanism enabling the steering of wheels independently of a driver's steering action.
Vehicle stability control devices as described above are directed to the reducing and/or canceling of effects of a braking or traction force imbalance owing to a difference of road conditions that wheels of a vehicle contact on, successfully improving its straight-line stability. However, the operations of the above-mentioned stability control devices are unexpectedly affected by the force imbalance arising from a vertical load shift in the lateral (left and right) direction of a vehicle such as during the turning of a vehicle. For instance, a vertical load shift owing to centrifugal force in a turning vehicle increases the braking or traction force on its turning outside wheels. Thus, even on a uniform road surface, if determined simply based upon the actually applied braking or traction forces, the counter yaw moment, and in turn, the automatic steering amount, would be generated unexpectedly in view of the purpose of the stability control, deteriorating a course tracking ability of a vehicle.
Accordingly, conventional vehicle stability control devices as described above may be improved to appropriately operate, taking into account a vertical load shift in the lateral direction of a turning vehicle.